Compaction means compressing powdery materials to greater density and smaller volume. Compaction is a common material-technological procedure in many fields of technology, particularly in metallurgy and ceramics. Compaction usually precedes a further phase, such as sintering.
In the pharmaceutical industry, compaction is used especially in making granules, particularly in dry granulation, when granules are made from moisture-sensitive materials. The pieces made during compaction are broken down into granules of desired shapes and sizes. Granulation, on the other hand, is one of the most important part processes of tabletting in pharmaceutical technology. Roughly speaking, granulation methods can be divided into wet and dry methods, depending on whether liquid is added to the powder mass or not. The purpose of granulation is to produce, from the powder, granules of appropriate size and strength in the shape of grains or balls. More broadly speaking, granulation means enlarging the size of powder particles. In granulating powders or powder compounds the aim is, for example, to improve the flow and distribution accuracy of the mass, reduce dusting, improve binding when compressing tablets, reduce separation of components, and accomplish a certain microstructure for the compacted powder.
Compaction is usually carried out as a batch process, but can also be performed as a continuous process, for example, by means of a roller compactor. Compaction can be carried out at room temperature or at an elevated temperature. The pressures used normally vary within the range of 70-700 MPa.
The properties most closely related to the behavior of the powder being compacted are the compressibility and green strength of the piece. Compressibility is an indicator of the change in density obtainable through compaction. Green strength is an indicator of the cohesion of the compacted piece.
A common problem of compaction processes is that the powder to be compacted does not become compressed in the expected manner. Especially where the compacted product is to be further worked into granules, it is a considerable disadvantage if loose powder has remained inside the compacted product, because this produces reject in the granulation process. If, on the other hand, the product to be compacted should remain in one piece as it is, for example, for the purpose of sintering or some other work phase, it is extremely disadvantageous if the compacted product splits during the compaction process.
One reason for the compaction problems is that the wrong compressing pressure has been selected with respect to those properties of the powder to be compacted that have an effect on compressibility. The most important properties affecting the compressibility of an organic powder are the capability of the powder to form van der Waals' bonds, particle size and shape, and particle size distribution. The appropriate compression pressure is dependent mainly on these properties. Different powder types are examined on laboratory and pilot scale, in an attempt to find the suitable compression pressure for each powder and powder compound type, with a view to the production process. In practice it has, however, been found that selecting the compression pressure merely on the basis of these parameters will not give a satisfactory compaction result. Thus, in practice, extensive series of test compressions have to be made with the powder or powder compound to be compacted in order to find the suitable compression pressure for the production process. Preparing such test batches is obviously laborious and very slow.
The aim of the present invention is to eliminate the above problems and to present a new method and device for monitoring the compressibility of a powder being compacted during the compaction process, and possibly for controlling the compaction process. The invention is based on the observation that acoustic emission gives a good indication of compressibility and thus the invention is based on analyzing the acoustic emission produced during the compaction process. If the powder is an organic substance, or a compound of organic substances, the acoustic emission arising from compressibility occurs at least for the most part in the audible region.
On the basis of the publication M J Waring et al, Int. J Pharmaceutics, 36 (1987) 29-36 it is known that the compressibility properties of pharmaceutical powders can be deduced on the basis of acoustic emission. Compressing has been studied in connection with tabletting by measuring the total amplitude of the acoustic emission as a function of time in the ultrasonic region. The publication does not analyze the spectrum of acoustic emission, and thus does not give any suggestions as to how the observation made could be utilised to control the compaction process.
Patent Publication No. 0 347 303 discloses the examination of the compaction of uranium pellets by means of a piezoelectric sensor, in which case the acoustic emission signal appears in the ultrasonic region. The occurrence and spreading of compression errors can be followed as a function of time on an amplitude curve. The spectrum of the acoustic emission has not been analyzed in this publication either.
Therefore, it has not previously been disclosed how the compaction process could be monitored and controlled on the basis of acoustic emission. A study of the literature did not bring forward any mention at all of measuring acoustic emission occurring in the audible region in connection with studies on the compressing properties of materials being compacted.
The compaction of organic substances is based on the fact that van der Waals' forces may cause binding between powder particles, the distance between which is smaller than 1000.ANG.. Binding improves if the number of connection points between the powder particles increases.
Friction work together with possible powder particle fractures give rise to acoustic emission when powder is compressed. The sounds are produced mainly in the audible region if organic substances are in question. The general properties of the substance, such as the ability to form van der Waals' bonds, and the specific properties of the substance batch in question, such as particle shape and size and particle size distribution, are the most significant parameters affecting compressibility.